Specific interactions between proteins and nucleic acids are involved in the control of gene expression, chromosome processing, the assembly of viruses, and genetic recombination. Bacteriophage systems as those of lambda and T7 are considered suitable for the study of these phenomena in biochemical depth due to an extensive genetic background available, and recent successes in reproducing complex reactions in cell-free systems. The developmental program of phage lambda depends on the action of the positive regulator pN, the product of the phage N gene. The mode of action of pN is to be investigated in a combined genetic and biochemical approach. The central issues concerning pN action to be studied include the effect of this protein on RNA polymerase and transcription-associated factors such as rho, the processing of this protein to give active and possibly inactive peptides, and the role of host factors (defined by mutation) in this interplay. Another area of protein-nucleic acid interaction is the assembly of the DNA-containing icosahedral head of phage lambda. Two aspects of the problem will be pursued - prohead assembly and the packaging of the phage chromosome into the prohead. Both reactions can take place in vitro and are subject to biochemical dissection and reconstitution. At both stages, phage and host-coded factors play a role, and the action of these factors is to be examined by means of a genetic and biochemical approach. Generalized recombination between phage T7 chromosomes takes place with high efficiency in cell-free extracts. The detection of crossing-over is possible because recombinant chromosomes can be assembled into virions in vitro and selectively scored as such. T7-specific functions, defined by genetic analysis, will be fractionated biochemically. Physical techniques will be used to elucidate the structure of recombinational intermediates made in vitro.